During the period in which the saving of resources has been eagerly discussed following the so-called 2nd Oil Crisis, many new technologies such as melting or heat decomposing furnaces and the technology for recycling plastic from waste plastic with the aim of resource recovery have been widely developed.
However, the reduction process employed in the melting furnace, heat decomposing furnace or the like has involved the consumption of several times the amount of energy which may be recovered thereby and, thus, the reduction process required more than three or four times the expense of the incinerating process in a conventional incinerator and its stable operation has been found to be difficult. The above reduction process has, thus, in fact, resulted in failure and many enterprises have ceased their efforts in this direction.
Further, in cases where the recycling of waste plastic is performed, it is difficult to sort the waste plastic since the many kinds of waste plastics are mixed together and, therefore, the recycled plastic contains a mixture of many kinds of plastics whereby the recycled plastic cannot be used in a manner similar to that in which fresh plastic material is used except in the case of making relatively low grade goods. Also, there is another problem in that harmful substances are contained in the plastic, such as heavy metals including stabilizing agents and pigments, etc. and, therefore, the usage of recovered plastic has been limited to the manufacture of goods such as stakes and flower pots which need not necessarily be made of plastic, and so the manufacturing thereof is, in fact, expensive and unprofitable.
Further, if the waste plastic is used for reclamation, since it is bulky because of its low specific gravity, and also since its volume does not reduce due to its relatively stable characteristics which tend to inhibit decomposition, it is difficult for the waste plastic to be effectively disposed of in reclamation.
As noted above, there have been difficulties in recycling waste plastic and disposing of the same such as in reclamation, etc. and there has thus been no suitable way of disposing of these waste plastics. It has therefore been desired to find a good solution for disposing of waste plastics and determining the proper pre-sorting requirement for collecting waste plastics which has been problematic for the workers concerned with garbage disposal in local government.
In some municipalities, waste plastics have been sorted out as being improper for incineration since some kinds of plastics will generate high temperatures which may damage a furnace if subjected to incineration in a mechanical furnace or the like such as a stoker. However, it is difficult to completely sort out plastics from the inflammable municipal waste and it is inevitable that a certain amount of plastic will remain in the waste after sorting, at least say 10% thereof.
Particularly in the case where vinyl chloride or the like is incinerated hydrogen chloride is generated which is apt to corrode metals and concrete, etc., thereby not only damaging the furnace or smoke stack but also becoming the cause for pollution. Great expense and effort has therefore been required in the disposal of the discharged gas generated by the incineration of vinyl chloride, etc.
In November, 1983, it was announced that the generation of harmful dioxin had been detected in the mechanical stoker type of incinerator and such announcement caused social unrest. The generation of organic chloride compositions such as polydioxin chloride (PCDD), polydibenzofuran chloride (PCDF) and benzpyrene, etc. give rise to a big controversy relating to the incinerating process and the main cause of the generation of harmful substances is regarded as lying in the existence of waste plastics. Thereafter, many research institutes have conducted studies on analyzing methods and effects on health but many unsolved matters remain regarding dioxin, etc. and the mechanisms whereby dioxins are generated and decomposed have not yet become clear.
However, according to the following equation [1], ##STR1## it is known that dioxin is generated by a de-hydrochloric acid reaction when a chlorophenol is heated to around 300-700.degree. C.
There are many mixtures in municipal waste, namely, plastics such as vinyl chloride, etc., various kinds of dyes, and chlorobenzene such as insect repellent. It is presumed that, when these many organic chemical substances are incinerated, a part thereof may become chlorophenol which causes the generation of dioxins. Accordingly, it has been desired to provide a non-polluting process for incinerating waste without producing such harmful substances as noted above.
It is said that dioxins are generally produced at a temperature below 700.degree. C. and they are completely decomposed by an oxidization process at approximately 900-1200.degree. C. Therefore, there are two processes for preventing the generation of dioxins, one being a decomposition process wherein the dioxin generated at the low temperature zone in the lower portion of an incinerator is decomposed by oxidization at 900-1200.degree. C. in a free-board of the upper portion of the incinerator, and the other being a high temperature process for completely preventing the generation of dioxins by constantly maintaining the temperature of the lower part of the incinerator at over 700.degree. C. (preferably over 800.degree. C. from the viewpoint of possible temperature variations and with a view to keeping the operating temperature at a safe level).
In cases where the furnace or incinerator is a mechanical type, it is only possible, due to its incineration mechanism, to employ the former process and the interior of the waste piled on a stoker at the bottom of the furnace is in the calcining state at 300-400.degree. C. corresponding to the range wherein dioxins are generated to the greatest extent. Further, a combustion air ratio in the mechanical furnace is as high as more than 2 and, therefore, the cooling rate by air therein is relatively high and, thus, it is difficult to raise the temperature at the upper portion of the incinerator to 900-1200.degree. C. unless the exothermic calory level of the waste to be burnt is relatively high.
For this reason incineration by using a fluidized bed was conceived. In the case where a fluidized bed type incinerator is employed, a fluidized bed comprising a fluidizing medium such as silica sand is formed at the bottom of the incinerator and this bed is usually operated at around 700.degree. C. Therefore, a process using such a fluidized bed is considered to be preferable in this regard compared to a process using a mechanical furnace. However, a complex chemical reaction zone exists within the fluidized bed in a fluidized bed type incinerator and it cannot be guaranteed, even with the use of this fluidized bed type incinerator, that generation of dioxins or the like will be effectively prevented at temperatures of approximately 700.degree. C. Therefore, it is advantageous to operate such an incinerator by further raising the temperature of the fluidized bed (preferably to over 800.degree. C.) while maintaining the temperature of the free-board at around 900-1200.degree. C. because the generation of organic chloride compounds such as dioxins is prevented with certainty, as well as the fact that other harmful substances such as PCB and cyan, etc. are almost completely decomposed at temperatures around 1200.degree. C.
On the other hand, disposal of industrial waste and various kinds of waste slurries, etc. that have been discharged from factories for reclamation purposes is becoming difficult and it has recently become a gradually growing trend to incinerate such wastes. Specifically, a fluidized bed type incinerator that can completely dispose of several kinds of wastes is becoming widely used and attempts are being made to incinerate waste that contains compounds of alkaline metals in the fluidized bed (of these incinerators).
However, a fluidized bed type incinerator commonly used for incinerating waste employs as a fluidizing medium silica sand (SiO.sub.2) having a mean grain size in the range between approximately 0.4 and 2.0 mm and incinerates the waste with thermal energy being added thereto together with the assistance of fuel, if necessary, the incineration taking place within a fluidized bed formed by the medium while the bed temperature is maintained at approximately 700.degree. C., and the thermal energy generated by the incineration being returned to the fluidizing medium. However, the silica sand which represents the fluidizing medium reacts with the alkaline-metal compounds, etc. as noted below and produces water glass, i.e. sodium silicate (Na.sub.2 O.3SiO.sub.2), which makes it impossible for the medium to be fluidized. It has therefore been the practice to limit the temperature of the fluidized bed in accordance with the kinds of object waste to be incinerated. EQU 3SiO.sub.2 +Na.sub.2 CO.sub.3 .fwdarw.Na.sub.2 O.3SiO.sub.2 +CO.sub.2 EQU 3SiO.sub.2 +2NaOH.fwdarw.Na.sub.2 O.3SiO.sub.2 +H.sub.2 O EQU 3SiO.sub.2 +2NaHCO.sub.3 .fwdarw.Na.sub.2 O.3SiO.sub.2 +H.sub.2 O+2CO.sub.2
That is, in the case where the weight contents ratio of the alkaline metal compounds represented by Na element (hereinafter referred to as Na-density) relative to the amount of the fluidizing medium (SiO.sub.2) is below approximately 0.5% (as in the usual type of municipal waste), the maximum temperature should be 800.degree. C., while in the case where the Na density in the fluidizing medium is as high as 1% such as in waste slurries or industrial waste which have a high content of alkaline-metal compounds, the temperature is to be maintained below 750.degree. C., which is the highest possible limit for keeping the operation at a safe level.
Incidentally, it is possible to incinerate waste containing alkaline-metal compounds, etc. in the fluidized bed incinerator by adding certain kinds of agents, for example, kaoline which suppress the potential for fusion of the silica sand in order to suppress the reaction between the sand and Na.sub.2 CO.sub.3 or NaOH. However, there is still a limitation in regard to the feeding density of alkalinemetal compounds, etc. in the waste relative to time and relative to the amount of fluidizing sand held within the incinerator, and fluidization stops even if the amount of suppressing agent is increased whenever the feeding density goes beyond the limit.
On conducting tests by use of an experimental furnace of the fluidized bed type, the Na density within the fluidizing sand was approximately 0.6-1.8% when fluidization of the fluidizing medium (SiO.sub.2) stopped at approximately 800.degree. C. due to the presence of alkaline-metal compounds in the combustibles, the variation being dependent on the kinds of alkaline-metal compounds and the fusion suppressing agents used.
Also, addition of a fusion suppressing agent is not effective in a case where the grain size of the agent is fine as this causes scattering of the agent at the time it is being charged into the incinerator, but it is also disadvantageous in that addition of the agent is expensive and increases the load that will be applied on the ensuing facilities. Consequently addition of such agents is not a powerful countermeasure capable of overcoming the problem induced by the presence of alkaline-metal compounds.
According to the test results obtained in the experimental furnace, the alkaline-metal compounds within the combustibles rae almost totally in the state of dust and mist in the exhaust gas when incineration is performed within the fluidized bed and discharged from the furnace. Thus the amount of alkaline-metal compounds remaining (accumulating) in the fluidizing sand is small and the amount of alkaline-metal compound remaining in the sand is mostly dependent on the amount of alkaline-metal compounds supplied together with the combustibles. Incidentally, not only is Na or K selectively trapped as constituents remaining in the sand but also other constituents, for example Fe and Ca, etc. may be present in a ratio corresponding to the ratio of the constituents as charged.
Because of the complex reaction behavior discussed above, in a case where waste containing alkaline-metal (Na, K) compounds is incinerated in the fluidized bed, a tentative standard for limitation of the processable density of (Na +K) is set as 1.0%/H with respect to the Na load (K also being converted to Na equivalent values) relative to the fluidizing sand. In fact, the acceptable density for Na and K may somewhat vary depending on differences in the forms of the alkaline-metal compounds (Na.sub.2 CO.sub.3, NaOH, NaCl, Na.sub.2 SO.sub.4 ; or K.sub.2 CO.sub.3, KOH, KCl, K.sub.2 SO.sub.4, etc.) but, since it is assumed that industrial waste includes a variety of alkaline-metal compounds due to its nature, the value 1.0%/H is set as Total-Na.
It is known that, in a case where plural kinds of alkaline-metal compounds co-exist, a co-melting point thereof is derived and such mixture melts at that point which is lower than the melting point of each individual alkalinemetal compound. This matter is of great importance and care must be taken in controlling the operation of a fluidized bed. In particular, amongst the alkaline-metal compounds contained in waste, Na.sub.2 CO.sub.3 or NaOH, etc. couple with the fluidizing sand chemically to produce sodium silicate or the like and, thus, care must without fail be taken with respect to controlling the amount of Na and K charged, as well as the temperature of the fluidized bed.
Under the circumstances discussed above, it has been required, for example, to introduce water into the fluidized bed or to recover thermal energy therefrom in order to keep the temperature of the fluidized bed below 800.degree. C. in the case where, for example, many plastics are contained in the waste and the thermal energy generated therefrom is large enough to raise the temperature of the fluidized bed beyond 800.degree. C.
Further, from the viewpoint of the combustion rate within the bed representing the ratio of incineration thermal energy to be returned to the fluidizing medium, silica sand generally belongs to the category of substances having a relatively low thermal conductivity since the thermal conductivity of the silica sand is low and is approximately 1.2 (800.degree. C.) Kcal/mh.degree. C.
There have therefore been occasions when the temperature of the fluidized bed cannot be maintained at a high level when the thermal energy derived from the waste is low.
Incidentally, if the combustion rate within the bed is low, the incineration ratio of the waste in the freeboard is increased and the mixing with the air therein is degraded such as to lower the total combustion ratio whereby the amount of harmful substances exhausted is increased.
On the other hand, some of the several kinds of waste slurries, waste liquids and industrial waste, etc. discharged from certain kinds of factories may involve many alkaline-metal compounds in an amount larger than that contained in municipal waste or waste plastics and may further include phosphide and vanadium compounds.
With respect to the temperature of a fluidized bed adapted to incinerate waste containing a large amount of the compounds referred to above, there is a certain limitation which corresponds to the content of those compounds. Depending on the kind of compounds concerned, it may be impossible to raise the temperature range beyond the 600.degree. C. level that is required for the incinerating process or it may be necessary to control and regulate the amount of alkaline-metal, phosphorus or vanadium within the fluidizing medium at a level below 1% by weight in a manner similar to the cases explained above, this being done by charging the waste gradually.
That is, particularly in the case where a large amount of alkaline-metal compounds, phosphides or vanadium compounds is contained within the combustibles, they or their oxides, etc. physically or chemically combine with the silica sand that serves as a fluidizing medium so that it exhibits a clinker. state which causes the bed to become inoperable. It has therefore been necessary to control the temperature of the fluidized bed below 600.degree. C. by decreasing the amount of comtustibles charged or introducing water into the fluidized bed.
However, incineration of waste by the fluidized bed has in most cases been conducted at an incineration temperature in the range of 600-800.degree. C. and, in the case where a large amount of alkaline-metal compounds is included within the waste, the temperature control referred to above has been essential, which has meant that such operations have in practice been impossible.
The present invention is directed to the incineration and disposal of several kinds of wastes by employing a fluidized bed and provides a method for processing the waste in a fluidized bed wherein grains of TiO.sub.2, or Al.sub.2 O.sub.3 are employed, instead of the silica sand conventionally used as a fluidizing medium, whenever the combustibles to be disposed of contain relatively large amounts of alkalinemetal compounds, phosphides or vanadium compounds, and a smooth incineration process is thus made possible even when the temperature in the fluidized bed is beyond 600.degree. C.